1 Introduction

Inorganic phosphates doped with rare earth activators can form important class of phosphors as they possess a lot of interesting characteristics, for example high luminescence efficiency, excellent chemical stability and flexible emission colors with different activators [1, 2]. The basic building block of phosphate compounds is PO4 tetrahedron which is flexible and can inhibit various coordination environments by altering the P–O bond distances. Moreover, they have many advantages such as high chemical stability, simple preparation method, structural diversity, and suitable band gap, which makes them suitable as the host for rare-earth activors. Recently, study on phosphate based luminescent materials have produced a large amount of literatures, such as K3Gd(PO4)2:Sm3+ [3], K3Gd(PO4)2:RE3+ (RE = Eu, Tb) [4], KBaBP2O8:Dy3+ [5], etc.

Among these, a series of orthophosphates with the general formula Na3Ln(PO4)2 (Ln = trivalent rare-earth metal) have been extensively studied for their good self-activated photoluminescence properties. This family of compounds are based on a common underlying Na3La(VO4)3-type structure with the orthorhombic space group Pbc21 (No. 29) and unit cell a ≈ 5.522 Å, b ≈ 14.05 Å, c ≈ 18.44 Å [6]; they are built up on isolated LnOx polyhedra, NaOx polyhedra and PO4 tetrahedra. Since the 1980s, a large amount of literatures concerning the luminescent properties of Na3Ln(PO4)2 compounds have been reported [7,8,9]. But until now, the detailed crystal structure of Na3Ln(PO4)2 family has not been satisfactorily described. Our previous work revealed that compound Na3La(PO4)2 features a four-dimensional incommensurately modulated structure with the super space group Pca21(0 0.387 0)000 [10]. For Na3Dy(PO4)2, the structure is still not known and its luminescent properties have not been concerned until now.

2 Experimental section

2.1 Materials and instrumentations

Raw materials Na2CO3 (≥ 99.8%), Dy2O3 (≥ 99.9%), and NH4H2PO4 (≥ 99.5%) were purchased from the Sinopharm Chemical Reagent Shanghai Co., Ltd. and were used without further purification. The powder X-ray diffraction (XRD) analyses were fulfilled on a Rigaku DMax2500 PC powder diffractometer with using graphite-monochromated Cu Kα characteristic radiation in the range of 2θ = 5°–60° (0.02°/step). IR spectra were recorded on a Magna 750 FT-IR spectrometer as KBr pellets in the range of 4500–400 cm−1. The morphology of the samples was examined by scanning electron microscopy (SEM) images taken using a ZEISS Merlin Compact scanning electron microscope. Photoluminescence (PL) spectra and the lifetime test were performed using an FLS920 Edinburgh Analytical Instrument apparatus. The steady-state measurements were performed using a standard 450 W continuous-wave xenon lamp as the excitation source. The step width 1 nm and integration time 0.2 s were used for the PL excitation and emission spectra measurements. The lifetime measurement was fulfilled by using a standard microsecond flash lamp μF920H with the time-correlated single-photon counting technique. The flash lamp operated at 200 Hz pulse frequency with a pulse width of 2 μs.

2.2 Syntheses

Powder sample of Na3Dy(PO4)2 was prepared by firing in a Pt crucible, raw materials (Na2CO3, Dy2O3, and NH4H2PO4) with the stoichiometric ratio, at 450 °C for 2 h and again at 800–1200 °C for 20 h. The purity of powder samples annealed at different temperatures can be confirmed by XRD analysis, as shown in Fig. 1. Single crystal of Na3Dy(PO4)2 was grown by molten salt method with excess Na2O–P2O5 as flux. The mixture with appreciate ratio Na/Dy/P = 25/1/15, was heat treated at 1100 °C for 20 h to ensure the best homogeneity, cooled at 3 °C/h to 800 °C, and finally cooled to ambient temperature by turning off the furnace. Finally the flux attached to the crystals was readily dissolved in hot water, and then a suitable crystal was carefully selected and mounted in a fiberglass for SC-XRD analysis.

Fig. 1
figure 1

XRD patterns of the Na3Dy(PO4)2 experimental samples heat treated at different temperatures (800–1200 °C) compared to the simulated data from single-crystal data

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2.3 SC-XRD analysis

A gray prism-shaped single crystal of Na3Dy(PO4)2 (0.20 mm × 0.05 mm × 0.05 mm) was selected from the reaction products. The crystal was mounted on a Bruker smart Apex2 CCD [11] and the data were collected by using a graphite-monochromatic MoKα radiation (λ = 0.71073 Å) at 293 K. Unit cell parameters were derived from a least-squares analysis of 728 reflections in the range of 2.18 < θ < 24.39. The data set was corrected for Lp factors, air absorption and absorption due to the variations in path length through the detector faceplate. Absorption correction based on Multi-scan method was also applied. All calculations were performed with programs from the SHELX-2013 crystallographic software package [12]. The structure was solved by direct methods. The crystal structure of Na3Dy(PO4)2 was solved in space group C2/c (No. 15) and all the atoms were located by subsequent cycles of refinements and Fourier difference maps. The final full-matrix least-squares refinement was on F 2 with data having F 2 ≥ 2σ(F 2) and all of the atoms were refined with anisotropic thermal parameters. The final R = 0.036, wR = 0.092 (w = 1/[σ2(F 2) + (0.0566P)2 + 9.9773P], (Δρ)max = 2.50, (Δρ)min = − 1.51 e/Å3 and S = 1.02 for 4221 observed reflections (I > 2σ(I)) with 194 parameter (Table 1). The important bond distances and selected angles for Na3Dy(PO4)2 are listed in Table 2.

Table 1 Summary of crystal data and structure refinement of Na3Dy(PO4)2
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Table 2 Selected bond distances (Å) and angles (°) of Na3Dy(PO4)2
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3 Results and discussion

3.1 Structure twinning of Na3Dy(PO4)2

It is well-known that twinning is the oriented association of two or more crystals of the same phase in some definite mutual orientation, and pairs of crystals are related by a geometrical operation termed twin operation, which is a symmetry operation for the twinned edifice but not for the crystal lattice [13, 14]. Non-merohedral twinning is a type of twinning that the twin operation does not belong to the Laue group or to the point group of the crystal, but usually belonging to a higher-symmetry supercell. This will lead to the overlap of certain zones of reciprocal lattice and thus the ordinary indexing may met with difficulties that the correct lattice parameters are impossible to be sure. In most cases, if we come to that problem, we can carefully split the twin pairs physically to get a single crystal. However, this method does not always work when the twinned crystal is ‘micro-twin’ rather than ‘macro-twin’. This time, we selected several crystals of Na3Dy(PO4)2 to perform SC-XRD analysis but found that they are all twinned, and a physical cut is void. Since crystals of Na3Dy(PO4)2 was synthesized by high temperature method, we temporary put forward that the twinned crystals of Na3Dy(PO4)2 was formed in the process of cool down to room temperature. This cause leads to all prepared crystals were micro-twinned.

As shown in Fig. 2, experimental reflections can be manually selected and assigned to two twin domains, leading to two almost identical orthorhombic unit cells which are related by an operator of 180° rotation about the (1 0 0) direction of reciprocal lattice. The twin law was confirmed to be a 3 × 3 matrix of %(1 0 − 0.097 0 − 1 0 0 0 − 1), and then a ‘HKLF 5’ type file can be generated instead of ‘HKLF 4’ file for refinement. The following resolution of twinning crystal is analogous to that of single crystal except inserting a BASF instruction. The final refined BASF parameter is 0.051 indicating that the twinning ratio for domain A: B is 0.949:0.051. The good refined parameters as mentioned above indicated that our twinning structure model of Na3Dy(PO4)2 is reasonable.

Fig. 2
figure 2

Reciprocal lattice viewer overlay tool for separation of reflections from different domains

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3.2 Structure description

Na3Dy(PO4)2 crystallizes in a monoclinic C-centered space group C2/c, and features a three-dimensional (3D) network containing PO4, NaO6, NaO7, NaO8 and DyO8 polyhedra. Interestingly, although the space group and unit cell of Na3Dy(PO4)2 differ greatly with our previously reported compound Na3La(PO4)2 [10] with a incommensurately modulated structure, the 3D structure are very similar. As shown in Fig. 3, All of the structures can be considered as an ordered superstructure of K2SO4 [15] in which K atoms are substituted by Na and Ln (Ln = La, Dy) atoms in the molar ratio of 3:1. For K2SO4, the symmetry of the structure requires that the successions of –K–SO4– and –K–K– adopt linear array. For Na3Ln(PO4)2, corresponding –Na–PO4– and –Na–Ln– successions adopt a bent configuration, which is more favored than the linear configuration to fit on the chemical bonding. As a result, the higher geometry of K2SO4 reduces to monoclinic geometry for Na3Dy(PO4)2, or a more complicated modulated structure for Na3La(PO4)2.

Fig. 3
figure 3

a Crystal structure of Na3La(PO4)2 viewed in the 8 × b supercell to show the linear array of –Na–La– and –Na–P–; b crystal structure of K2SO4 to show the linear array of –K– and –K–S–; (b) approximant cell of incommensurately modulated; c crystal structure of Na3Dy(PO4)2 to show the linear array of –Na–Dy– and –Na–P–

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There are five unique Na atoms, two unique Dy atoms, three unique P atoms and twelve unique O atoms in the asymmetric unit of Na3Dy(PO4)2. Each P atom is coordinated by four oxygen atoms into PO4 tetrahedron with the P–O bond distances of 1.5131(2)–1.5409(3) Å and O–P–O bond angles of 105.525(6)°–111.795(6)°, as listed in Table 2. One can notice that the deviation of PO4 tetrahedron from regular tetrahedron is negligible, which is comparable with other reported phosphates [16, 17]. Furthermore, each PO4 tetrahedron connects with eight NaOx (x = 6, 7, 8) and three DyO8 groups into a 3D network of compound Na3Dy(PO4)2. The calculated total bond valences for Dy1, Dy2, P1, P2, P3, Na1, Na2, Na3, Na4 and Na5 atoms are 2.796(11), 2.940(13), 4.85(3), 4.87(3), 4.92(4), 1.118(5), 1.076(5), 0.971(4), 1.040(4), 1.041(5), respectively, indicating that Dy, P and Na atoms are in reasonable oxidation states of + 3, + 5 and + 1 [18].

3.3 IR spectroscopic and morphology

The IR spectrum of the compound Na3Dy(PO4)2 is shown in Fig. 4. To assign the IR peaks to vibrational modes, we examine the modes and frequencies observed in similar compounds [19]. The broad and intense absorption bands appearing at 990–1150 cm−1 are characteristic of P–O stretching vibration in the PO4 tetrahedron, band group at 470–590 cm−1 is bending vibrations of P–O bond, and band at 450 cm−1 is due to Dy–O and Na–O vibrations.

Fig. 4
figure 4

IR spectrum of Na3Dy(PO4)2 ranging from 400 to 4500 cm−1

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Figure 5 shows the SEM images of Na3Dy(PO4)2 annealed at different temperatures. It is clearly seen from the image that all samples are crystalline with irregular shape, narrow size distribution and block morphology, which reveals the inherent characteristics of the adopted solid state method. The broad size distribution of particles may be caused by processing the materials through grindings. The other salient characteristic is that the morphology of the sample depends on the calcination temperature heavily. With the increasing of annealed temperatures, the largest sizes of particles increase gradually: less than 10 μm for samples calcined at 800‒900 °C, and more than 20 μm for for samples calcined at 1000‒1200 °C. Obviously, the degree of crystallinity has vastly improved when the calcined temperature increases from 900 to 1000 °C. However, no further improvement is made when the calcined temperature is > 1000 °C.

Fig. 5
figure 5

SEM micrographs of Na3Dy(PO4)2 calcined at temperatures 800 (a), 900 (b), 1000 (c), 1100 (d) and 1200 °C (e)

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3.4 Emission and excitation spectra

Dy3+-activated phosphors usually show strong fluorescent transitions in the blue (around 482 nm) and yellow (around 574 nm) emissions corresponding to 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions, respectively [20, 21]. Therefore, a white–yellow phosphor can be produced using Dy3+ ion as the sole activator by adjusting the yellow/blue intensity ratio to a suitable value. Figure 6a exhibits the excitation spectra of the samples Na3Dy(PO4)2 sintered at 1000 °C by monitoring the emission at 574 nm (4F9/2 → 6H13/2 transition of Dy3+ ions) in the range of 200–500 nm. There exists a broad band at 250‒280 nm, which can be attributed to the charge-transfer (CT) band from O2− to Dy3+ ions [22]. Besides, several sharp excitation peaks between 280 and 490 nm centered at 295, 325, 350, 365, 387, 427, 452 and 474 nm can be assigned to the 6H15/2 → 4L17/2, 6H15/2 → 4I11/2,6H15/2 → 6P7/2, 6H15/2 → 6P5/2, 6H15/2 → 6I13/2, 6H15/2 → 6G11/2, 6H15/2 → 4I15/2 and 6H15/2 → 4F9/2 transitions, respectively [23]. Clearly, all of the peaks are attributed to the intra-4f forbidden transitions.

Fig. 6
figure 6

Excitation (a) and emission (b) spectra of Na3Dy(PO4)2 sintered at temperature 800–1200 °C

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As shown in Fig. 6b, the emission spectrum is composed of several distinct groups of sharp lines in the range of 400–800 nm. Two weak emission bands centered at 669 and 755 nm are due to the 4F9/2 → 6H11/2 and 4F9/2 → 6H9/2 spin-forbidden transitions of Dy3+. Besides, there exist two emission regions at blue and yellow regions. The blue emission band at 483 nm corresponds to the 4F9/2 → 6H15/2 transition and the yellow emission band at 574 nm corresponds to the hypersensitive 4F9/2 → 6H13/2 transition. This behaviour indicates the presence of non-radiative processes depopulating 6P7/2 to 4F9/2, and the broadening of emission lines of Dy3+ are related to several Stark levels for the 4F9/2 and 6HJ levels. It is well-known that the 4F9/2 → 6H13/2 (yellow) electric dipole transition is hypersensitive and its intensity strongly depends on the nature of the host, while the intensity of 4F9/2 → 6H15/2 (blue) magnetic dipole transition is less sensitive to the host lattice. Afterworlds, the yellow-to-blue intensity ratio R, which is due to the integrated luminescence intensities of the 4F9/2 → 6H13/2 (yellow) and 4F9/2 → 6H15/2 (blue) transitions of Dy3+, is an important parameter to check the symmetry of Dy3+ ion, which can be calculated using the equation [10]:

$$R=\frac{{\int {I\left( {{}^{4}{F_{9/2}} \to {}^{6}{H_{13/2}}} \right)} }}{{\int {I\left( {{}^{4}{F_{9/2}} \to {}^{6}{H_{15/2}}} \right)} }}$$

The values were calculated to be 1.81 (excited by 350 nm), indicating that Dy3+ ion locates in low symmetry without an inversion center in the crystal lattice. This result fits well with the crystal structure analysis mentioned above that Dy3+ ion locates in a low symmetry site.

The influence of calcination temperature on photoluminescence was investigated for phosphor Na3Dy(PO4)2 because the calcination temperature is usually closely related to the crystallinity as well as luminescent properties of a phosphor. With the increasing of the annealed temperature, the PL intensity is enhanced from 800 to 1000 °C. The strongest emission intensity of the Na3Dy(PO4)2 phosphor annealed at 1000 °C is 2.73 times comparison to that at 800 °C. This may be ascribed to the increasing of the crystalline size with the raising annealed temperature, which gives rise to a decreasing of light scattering. This conclusion can be confirmed by morphology of samples annealed at temperatures 800‒1200 °C, as discussed above (Fig. 5). When annealed temperature rises from 1000 to 1200 °C, the crystallinity degree does not increase significantly, but more lattice defect may be formed for excessively high annealed temperature. As a result, the luminescent intensity decreases when annealed temperature is above 1000 °C.

3.5 Decay time and color chromaticity

The decay curve of Na3Dy(PO4)2 phosphor for the yellow emission at 574 nm (4F9/2 → 6H13/2) excited by 350 nm is measured, as shown in Fig. 7. It is found that after a rapid initial rise in the PL intensity, when irradiation at the wavelength of 350 nm was started, the PL intensity at an emission wavelength of 574 nm decayed slowly to a steady-state value of about micro-seconds. The decay curve cannot be well fitted with a single exponential but can be well fitted with bi-exponential function according to the equation:

Fig. 7
figure 7

Fluorescent decay (black) and fitting (red) curves of Na3Dy(PO4)2

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$$I = {A_1}^*\exp \left( { - t/{\tau _1}} \right){\rm{ }} + {A_2}^*\exp \left( { - t/{\tau _2}} \right)$$

where I is the luminescence intensitie; A 1 * and A 2 * are the fitting parameters; t is the time; τ 1 and τ 1 are the luminescent lifetime. The average decay time was calculated to be 2.11 μs to represent the lifetime by the equation, which is comparable with other reported Dy3+ activated phosphor [24].

$$\tau =\frac{{{A_1}\tau _{1}^{2}+~{A_2}\tau _{2}^{2}}}{{{A_1}{\tau _1}+~{A_2}{\tau _2}}}$$

Usually, the index of exponential functions agrees with the number of Dy3+ ion sites in the host lattice. We think that there are two decay phenomenon revealed by the experiment, meaning that the divalent dysprosium is located in two crystallographic sites, which is well fitted with the crystal structure data as mentioned above.

Figure 8 presents the commission international Del’Eclairage (CIE) 1931 color space chromaticity diagram of phosphor Na3Dy(PO4)2 annealed at 1000 °C under 350 nm excitation. The chromaticity coordinates were calculated to be (x = 0.3968, 0.4364), falling in the yellow region, which imparted that the material may be used as a yellow phosphor on lighting and displays.

Fig. 8
figure 8

The CIE chromaticity diagram for Na3Dy(PO4)2 with excitation at 350 nm

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4 Conclusions

This work provided a sodium dysprosium phosphate Na3Dy(PO4)2, and determined its crystal structure by single-crystal X-Ray diffraction analysis for the first time. Refinement of the structure is complicated by two components non-merohedral twinning. The relative orientations of the domains are related by a 180° rotation about the (1 0 0) direction of reciprocal lattice, thus giving the twin law of (1 0 − 0.097 0 − 1 0 0 0 − 1). Na3Dy(PO4)2 crystallizes in the monoclinic space group C2/c, and features a 3D framework that is constructed by PO4, NaO6, NaO7, NaO8 and DyO8 polyhedra. Although with fully Dy3+ concentration, the emission spectrum excited at 350 nm shows strong emission bands at 483 and 574 nm, corresponding to the characteristic 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions of Dy3+ ion. The decay curve at 350 nm was measured and the decay curve can be well fitted by bi-exponential function, which can be explained by two crystallographic sites of Dy atom in the structure. The calculated CIE color coordinates are (0.373, 0.379) which fall in the yellow light region of the color gamut. Therefore, the potential of Na3Dy(PO4)2 phosphor under UV excitation may be explored as a yellow phosphor for solid state lighting and displays. The study on the factors (including calcining time, sensitizing ion and synthetic method) that may greatly influence the luminescent intensity of Na3Dy(PO4)2 phosphor is in progress.